Apparatus and method for generating short optical pulses

ABSTRACT

An apparatus for generating short optical pulses is provided having a storage medium capable of storing optical energy, a first module for delivering pumping optical pulses or continuous beam into the storage medium to energize the storage medium, and a second module for delivering one or more trigger optical pulses to the optical storage medium. Each of the trigger optical pulses has a rising edge which causes a cascade release of the energy stored in the storage medium in an output optical pulse having a greater power and narrower in width or duration (at full width at half maximum) than the trigger optical pulse that caused the output pulse. A control module in the apparatus controls the operation of the pump module and trigger module so as to provide the desired characteristics of output optical pulses from the apparatus.

This application in a continuation of U.S. application Ser. No.11/546,236, filed Oct. 11, 2006, now U.S. Pat. No. 7,711,014, whichclaims priority to U.S. Provisional Application No. 60/724,880, filedOct. 11, 2005, which is herein incorporated by reference.

FIELD OF THE INVENTION

This invention relates to an apparatus and method for generating shortoptical pulses, and in particular to an apparatus and method forgenerating short optical pulses using an optical storage medium which ispumped with energy that is efficiently released by the rising edge oftrigger pulses as high power short optical pulses, rather than toamplification of entire trigger pulses. The optical pulse may be on theorder of sub-microsecond down to sub-nanosecond in temporal width. Theinvention is useful as an optical pulse source in systems and devicesfor LIDAR, remote sensing, laser altimeters, laser range finding, andmedical procedures.

BACKGROUND OF THE INVENTION

Short pulse, high power lasers are used in various applications, sincethese lasers have power advantage of a pulsed laser over a continuousoutput laser is that the energy output can be compressed into a veryshort time period, resulting in very high energy per unit time. Twotraditional ways of achieving short optical pulses with high opticalpower are cavity Q-switching and cavity mode locking. In the Q-Switchmethod the Q-factor or figure of merit for a cavity is initially setvery low such that energy transferred into the cavity does not induceappreciable stimulated emission. The energy within the cavity is allowedto build up until the Q-factor of the cavity is switched rapidly to avery high state such that significant feedback is present, significantstimulated emission is generated, and the cavity lasers, therebygenerating an intense, short optical pulse that discharges a substantialportion of the energy the cavity had stored during the low-Q state. TheQ-switch technique allows substantially more energy to be stored andreleased by the resonant cavity than if the resonant properties of thelaser cavity were not reduced by the Q-switch. The Q-switch may beaccomplished by the use of non-linear crystals, saturable absorbers, andoscillating or rotating mirrors. Q-switched lasers are described, forexample, in LASE 2004 Conference Proceedings 5332, and in J. Nettleton,et. al. “Monoblock laser for a low-cost, eyesafe, microlaser rangefinder”; Applied Optics, Vol. 39, No. 15, 20 May 2000, pp. 2428-2432.Cavity mode locking also utilizes a laser, but instead of a Q-switch,the longitudinal modes of the laser are locked to a set spectral spacingsuch that the superposition of the broad spectrum of individual spectralpeaks superimposes to create an optical signal that is narrow in thetime domain. Cavity mode locking is described, for example, in Siegman,LASERS, University Science Books, January 1986, ISBN 0935702113.

Although providing very short optical pulses, lasers operating byQ-switching and cavity mode locking are expensive, large and bulky, andare often custom built for optical applications. Thus, other ways havebeen developed for achieving short high power optical pulses using acladding pumped fiber. Cladding pumped fiber lasers use a speciallyprepared glass fiber having a core on the order of 5 micrometersdiameter doped with rare earth ions, such as Erbium (Er), Neodymium(Nd), or Ytterbium (Yb). The surrounding glass cladding which supportsthe doped core is irradiated longitudinally (along the fiber axis) byhigh power pumping lasers whose wavelength is selected to be absorbed bythe rare earth dopants, and whose combined power may be many kilowatts.The cladding is much larger in cross section than the core, so that muchmore optical power can be injected than could be injected directly intothe core. As the high power pumping laser beams cross the core of theglass fiber they are not captured to form a guided wave, but nonethelessare partially absorbed to energize the rare earth ions. The cladding isintentionally fabricated so that it is not round, and the pumped laserlight undergoes mode-mixing in the glass fiber to avoid depleting themodes that intersect the core. Using this method, IPG Photonics ofOxford, Massachusetts, USA, produces lasers that exceed 10 kiloWattscontinuous output in a beam diameter of 100 micrometers. See also forexample, U.S. Pat. No. 5,949,941, issued to D. J. DiGiovanni, titled“Cladding-pumped fiber structures”.

A different class of actively pumped fiber often used in opticalcommunication systems injects the pumping laser beam directly as asingle mode beam into the doped core of the glass fiber. This oftenlimits the amount of pump power that can be injected to many hundreds ofmilliwatts. Such a device is often referred to as an optical fiberamplifier, since it is typically used to amplify optical signals of theproper wavelength as they pass through the core of the fiber. Theseamplifiers are generally operated in the linear regime, where the inputsignal is small enough that the gain is independent of the signal. Inthis linear region, the output signal is an amplified exact replica ofthe input signal in terms of wavelength, polarization state, power vs.time. For example, such optical amplifiers are described in Waarts etal., U.S. Pat. Nos. 6,081,369, 5,933,271, and 5,867,305. The goal ofsuch optical amplifiers is to produce an output optical signal that isan amplified version of an input optical signal, and as such it isimportant to maintain fidelity of the temporal shape of the outputsignal with respect to the input signal. For the case of a series ofinput optical pulses, these optical amplifiers provide output opticalpulses having the same wavelength, pulse width, and spacing betweenpulses as the input optical pulses, but at an increased optical powerlevel. Although useful in optical communication systems to linearlyamplify communication signals, the release of stored energy from theoptical fiber must be over the entire input signal to linearly amplifysuch signals, which limits applications of such amplifiers.

Although optically pumped fiber are useful to transfer energy coherentlyinto optical signals, i.e. to optically amplify the optical signals theyhave not heretofore been used with trigger pulses as described by thepresent invention which causes a cascade energy release of stored energyin the form of high power optical pulses that more efficiently utilizethe stored energy, rather than to amplification of the entire opticalpulse that triggered such release. Thus, it would be desirable toproduce short high power optical pulses without the complexity andexpense of Q-switching and cavity mode locking based devices, which areinitiated by such optical trigger pulses applied to a pumped energystorage medium, where each trigger pulse produces a cascade energyrelease as an output optical pulse that is not an amplified replica ofthe trigger pulse or any other input optical signal.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anapparatus and method for generating short high power optical pulses byapplying one or more optical trigger pulses to an energy pumped storagemedium such that the medium outputs short duration optical pulses by acascade energy release of stored energy from the storage medium.

It is another object of the present invention to provide an apparatusand method for generating short high power optical pulses using triggerpulses applied to a pumped optical storage medium in which the opticalpulses output from the storage medium have applications in LIDAR, laserrange finding, laser altimeter, remote sensing, and medical uses.

Briefly described, the present invention embodies an apparatus having astorage medium capable of storing optical energy, a pump module fordelivering a series of pumping optical pulses or continuous beam into anoptical storage medium to energize the storage medium, a trigger modulefor delivering one or more trigger optical pulses in which each triggerpulse triggers a cascade release of the energy stored in the storagemedium in an output optical pulse having greater power and beingnarrower in width or duration (full width half maximum) than the triggeroptical pulse that caused the output optical pulse. A control module inthe apparatus controls the operation of the pump module and triggermodule so as to provide the desired characteristics of output opticalpulses, such as power, repetition rate, and pulse width or duration bytiming of pumping and trigger pulses, and wavelength(s) when the triggermodule contains one or more light sources of different or tunablewavelength(s).

The trigger optical pulses each have a rising edge which triggers therelease of each of the output pulses from the storage medium by causinga cascade release of energy stored in the optical storage medium in astimulated discharge of substantially all of the energy stored by astorage medium. This rapid discharge or dump of the energy in responseto the trigger pulse is as an avalanche effect, and thus the apparatusprovided by the present invention is called an avalanche pulsed laser(APL). The APL can generate optical pulses on the order of microsecondsto sub-nanoseconds, but preferably is less than one microsecond, or lessthan 10 nanoseconds depending on application of the output pulses. Thetrigger module consists of one or a plurality of trigger sources.Preferentially, laser trigger sources are utilized to generate a triggerpulse or series of trigger pulses that rapidly release the energy storedby the storage medium through stimulated emission. The power and shapeof the trigger pulses are such that the initial portions of the triggerpulse creates an exponentially increasing cascading energy release anddepletes the energy out of the APL such that the trailing portions ofthe trigger pulse are not significantly amplified. The result is a shortoutput optical pulse that can have orders of magnitude more power thanthe initial trigger pulse as well as having the output pulse besignificantly narrower than the initial trigger pulse. The triggermodule may have different trigger sources or wavelength-tunable triggersources to enable the APL to tune the output pulse wavelength or to emitalternating pulses of different wavelengths. Also, the shape of thetrigger pulses, such as their rising edge, provided by one or more lightsources of the trigger module may be controllable by the controllermodules.

The pump module represents a single pump source or a plurality of pumpsources for pumping energy into the storage medium. The trigger modulerepresents a single or a plurality of optical trigger sources thatdeliver an optical trigger pulse or sequence of optical trigger pulsesinto the storage medium.

Any device that is capable of storing the energy received from thepumping module and releasing the energy in the form of an optical beamwhen incited to by the trigger module may represent the storage medium.Examples of the storage medium include laser gas mixtures, such as HeNe,HeCd, ArF, semiconductors, such as GaAs, AlGaAs, and GaInAsP, or glassand crystals, such as Nd:YAG and Erbium-doped silica fiber, or photonicbandgap fiber. The storage medium is selected based upon parameters suchas the desired emission wavelength, energy storage efficiency of thestorage medium, and the ability of the storage medium to efficiently andrapidly emit its stored energy in an optical form.

One feature of the present invention is that the output pulse of the APLis significantly shortened in time in relation to the trigger opticalpulses. This represents optical energy discharge different from opticalgain, the property optimized in optical amplifiers. The input triggerpulse is not optically amplified, only a portion—this being the leadingedge of the trigger pulse—gains significant energy from the energystorage medium The leading edge rapidly builds to an intensity which hasextracted sufficient energy that the leading edges suppresses the gainfor the remainder of the pulse. Thus, the output is not only shorter intime, but is significantly different in temporal shape than the inputtrigger signal. In the case where the storage medium represents anoptical fiber, only the leading edge of the trigger pulse amplifiesexponentially in the early part of the doped fiber, leaving the gainincreasingly depleted as the pulse propagates, so that the trailing edgeof the pulse receives less gain. In the final section of the opticalfiber in which the pulse fully depletes the stored energy, the trailingedge of the pulse receives no amplification at all. Due to the depletionof substantially all of the storage medium's dischargeable energy, anyvariations of the trigger signal are lost as the energy is extracted, sothat no trigger signal variations are present in the output pulse. Inthis manner, the trailing edge of the trigger pulse is not significantlyamplified since it sees a substantially depleted storage medium and theresult is that the output pulse is narrower and contains significantlylarger power than the initial trigger pulse.

In the case that the storage medium of the APL is an optical fiber, theoptical fiber acts as an efficient light guide for the optical triggerpulse and may be utilized to guide optical radiation coming from thepump module such that the optical fiber storage medium can beefficiently pumped. Specific examples of optical fiber capable of beingused to generate optical pulses include Erbium-doped or a combination ofErbium and Ytterbium dopants in a silica fiber. These dopants allow foroptical gain in the 1520-1650 nm wavelength band. For these specificexamples, the pump module may be a flash lamp, but preferentiallycontains a single or a plurality of fiber-coupled lasers emitting atapproximately 980 nm or 1480 nm. For this example, the trigger opticalsource can be a fiber-coupled laser or other fiber coupled source, suchas an LED (Light Emitting Diode) emitting at a wavelength ofapproximately 1520-1650 nm. Since the APL works under the principle ofstimulated emission, the output wavelength of the APL is the samewavelength as the trigger optical source.

In the case of the storage medium of the APL is a crystal or glass, thecrystal or glass has an appropriate composition to provide opticalsignal amplification at the desired wavelength. An exemplary material isNeodymium doped YAG (Yttrium Aluminum Garnet), which provides opticalgain at a signal wavelength of 1060 nm. An alternative material is aphosphate glass doped with Erbium and Ytterbium, providing gain in thewavelength range of roughly 1520-1650 nanometers. This material can beused in bulk form with the light from the trigger sources coupled intoit through the use of bulk optics or the material can be in wafer form,where the material is patterned into waveguides onto wafer or wafers.

The trigger module may emit a pulse of a single spectral wavelength or aspread of spectral wavelengths. When emitting a trigger pulse of aspread of spectral wavelengths, the resulting pulse out of the storagemedium has the same spectral spread of wavelengths as that of thetrigger module. Since light of different wavelengths will propagate atslightly different velocities through a storage medium due to index ofrefraction dispersion, an optical element of a dispersive material ordispersive compensating material may be added to the output of thestorage medium in order to broaden or shorten, respectively, the outputoptical pulse. Such dispersive material or dispersive compensatingmaterial may for example be free-space gratings, fiber gratings, orfiber spools, or any combination thereof.

In case where the storage medium in an optical fiber, or a medium of asmall size such that the optical intensity of the transmitted opticalsignal may cause optical damage to the surface of the storage medium, anoptional transparent optical element of a bulk optical material may beattached to the output of the optical fiber. The purpose of suchtransparent optical material is to prevent optical damage that maypotentially occur at the exit face of the optical fiber when the opticalpulse initiated by the trigger module and generated within the storagemedium exits the fiber. By attaching a bulk transparent material to theoptical fiber end face, the optical pulse first exiting the fiber facewill expand substantially due to diffraction before exiting the air-bulktransparent material interface. The result being that the optical powerdensity of the optical pulse exiting the transparent bulk material isreduced versus that of the optical fiber if no transparent opticalmaterial is attached. To reduce back reflections that may create opticaldamage, the transparent optical material is preferentially index matchedto the optical fiber.

The APL may emit optical pulses at arbitrary wavelengths through theproper selection of pump sources, trigger sources, and storage medium.For the 1.55 μm wavelength regime, the storage medium is preferentiallyerbium-doped optical fiber, the pump sources are a combination of 980 nmand 1480 nm lasers, and the trigger sources are high-speed semiconductorlasers emitting in the telecommunication C and L-band (i.e., 1520-1650nm). This 1550 nm wavelength regime is advantageous for remote sensing,LIDAR, laser range finding, and laser altimetry applications due to itbeing considered eye-safe. This wavelength regime is also advantageousfor the medical community for cosmetic skin surgeries.

Also, the APL by use of polarized trigger pulses may output opticalpulses of polarized light, thereby enabling remote sensing applicationsthat employ heterodyne and homodyne detection techniques. Also, anoptional polarization control element may be provided to change thepolarization of the output pulses, such as TE or TM, or linear orcircular polarization. Such polarization control element may be staticor dynamic. Where a dynamic polarization control element is used, thecontrol module may control the polarization state of such element. Aninterferometric detection module may be provided within or external ofthe housing of the APL to enable heterodyne or homodyne detection of areflected output pulse from the APL.

A typical characteristic of optical energy storage media is spontaneousdecay emission which is the process by which an energized atom decaysfrom its energized state to a lower energy state, releasing a photonwithin the useful signal range. Such a photon is not differentiated bythe medium from a desirable trigger photon. Spontaneous decay photonsmay be amplified within the gain medium, producing Amplified SpontaneousEmission (ASE), which is a parasitic loss of energy from the energystorage medium (e.g., is not available to contribute to a desired outputsignal). If needed in order to suppress ASE in the APL, an opticalisolator may be provided in the optical path of the generated pulse toprevent backward traveling light. An alternate method to suppress ASE,is to place optical filter(s) in the path of the generated pulse; suchoptical filters transmitting the wavelength(s) of the trigger pulses,but substantially attenuate wavelength(s) which depart from the triggerpulses. The optical filters prevent the build up of out-of-band ASE inboth the forward direction and the reverse direction. In the case of aphotonic bandgap fiber as the energy storage medium in an APL, such afiber is built to allow only a narrow transmission around the desiredtrigger laser wavelength and acts as an optical filter along the entirelength of the fiber. This will similarly suppress the growth of ASE inthe APL, and thereby reduce the parasitic loss to ASE. The prevention ofthe buildup of ASE in both the forward and the backward direction allowsa greater amount of energy to be stored in the optical storage medium.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing objects, features and advantages of the invention willbecome more apparent from a reading of the following description inconnection with the accompanying drawings, in which:

FIG. 1 is a block diagram of the apparatus of the present inventiontermed an avalanche pulsed laser (APL);

FIG. 2 is a timing diagram illustrating the timing and power of the pumppulses and the trigger pulses to generate output pulses from the APL;

FIG. 3 is a schematic block diagram of one embodiment of the APL of FIG.1 having a doped silica fiber as the storage medium;

FIG. 4 is an energy level diagram of an Erbium-doped silica fiber ofFIG. 3 to illustrate the operation of the APL;

FIG. 5 is a block diagram of the end of the fiber of FIG. 3 with anoptional bulk transparent material optical element to reduce the opticaldensity at an air interface;

FIG. 6 is a block diagram of the APL similar to FIG. 1 having polarizedoutput optical pulses and a module for enabling heterodyne or homodynedetection in remote sensing applications; and

FIG. 7 is a block diagram of the APL similar to FIG. 6 in which themodule for enabling heterodyne or homodyne detection is located outsidethe APL housing.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a block diagram of the apparatus of the presentinvention, termed an avalanche pulsed laser (APL) 100, is illustrated.APL 100 has a control module 111, a trigger module 110, a pump module112, and a storage medium 113. The pump module 112 delivers energy tothe storage medium 113 which stores said energy, and trigger module 110delivers trigger pulses to the storage medium to cause a cascade releaseof such stored energy in output optical pulses 109, as described below.The control module 111 contain electronics for controlling the operationof the trigger module 110 and pump module 112 in response to externalcontrol signals received via a control cord or cable 102 to changespecific APL output pulse 109 characteristics, such as pulse power,pulse repetition rate, pulse wavelength, and pulse length. Power to theAPL 100 is supplied via a power cord 101 to control module 111.Optionally, the APL 100 may have an internal power source that by way ofexample may be a battery or a solar cell combined with a rechargeablebattery.

Power lines 103 and 105 and signal (or control) lines 106 and 104respectively connect the control module 111 to the trigger module 110and pump module 112. The signal lines 104 and 106 control the timing ofthe trigger module 110 and the pump module 112, respectively, which isused to change output pulse 109 characteristics. The pump module 112supplies energy to the storage medium 113 via a pump connection (orpath) 107 and such energy being supplied in an intermittent or acontinuous manner may be in one or more forms including, but not limitedto, visible light, infrared radiation, ultraviolet radiation,electricity, or heat. Pump connection 107 may have one or more opticalelements for communication of optical beam(s) to the storage medium 113,such as fiber optics, beam shaping lens(es), or other opticalcomponents, which may depend on the particular type of storage medium113. The pump module 112 may contain a single or a plurality of pumpsources that may include but are not limited to flash lamps, lasers,LEDs, heat sources (e.g., heated filaments), light emitting electroniccircuits, fluorescent lamps, and inductive heaters. By way of example,multiple pump sources may be desirable for the purposes of more rapidlystoring energy into the storage medium 113 or for the purpose of pumpingthe storage medium 113 with more than one form of energy. As aclarifying example, the storage medium 113 may be more efficientlypumped if it is exposed to more than a single wavelength of light or ifit is exposed to heat while simultaneously exposed to light. As afurther clarifying example, the storage medium may be pumped by theapplication of an electric current, for example in creating excited gasions in a Helium-Neon gas or in an Argon ion gas.

The storage medium 113 comprises any type of media capable of storingenergy such that such stored energy can be dumped into an optical beamthrough the process called stimulated emission. As such, the storagemedium 113 may be a media that includes but is not limited to gases,semiconductors, glasses, or crystals. For storage medium that includegases, examples include laser gas mixtures, such as HeNe (for APLemission wavelengths including 0.5435, 0.5941, 0.6119, 1.12, and 3.92μm), HeCd (for APL emission wavelengths 0.442 and 0.351 μm), and ArF(for APL emission wavelength of 0.192 μm). For storage medium 113 thatcontain semiconductors, examples include GaAs (for APL emissionwavelength of 0.84 μm), AlGaAs (for APL emission wavelength of 0.76 μm)and GaInAsP (for APL emission wavelength of 1.30 μm). For storage mediumthat include glass and crystals, clarifying examples include Nd:YAG (forAPL emission wavelength of 1.06 μm) and Erbium-doped silica fiber(wavelengths of APL emission that range from 1.52 to 1.65 μm), aphotonic bandgap fiber. Such an optical fiber of glass material may besilica glass, phosphate glass, fluoride glass, or chalcogenide glass, inwhich optionally at least a portion of such glass material has aconcentration of one of or more of Erbium, Ytterbium, Neodymium, orPraseodymium that forms a continuous filament or channel between ends ofthe optical fiber. For storage media that include liquids and organicmolecules, clarifying examples include Rhodamine 6G dye moleculesdissolved in ethanol.

The particular storage medium 113 is chosen based upon parameters suchas the desired emission wavelength, energy storage efficiency of thestorage medium, and the ability of the storage medium to efficiently andrapidly emit its stored energy in an optical form.

In the case of the storage medium 113 of the APL 100 is a photonicbandgap fiber, such a fiber has photonic bandgap structures or crystalsthat resemble an optical filter, in that there are wavelengths that willbe transmitted and wavelengths that will be blocked, but they can bemade substantially isotropic. Photonic bandgap fiber are described forexample, in Bjarklev et al., Photonic Crystal Fibers, published bySpringer (2003), ISBN 1-4020-7610-X, and also in The Encyclopedia ofLaser Physics and Technology, “Photonic Bandgap Fiber”,http://www.rp-photonics.com/photonic_bandgap_fibers.html. Since aphotonic bandgap fiber is built to allow only a narrow transmissionaround the desired trigger laser wavelength(s), it also may suppress thegrowth of ASE in the APL, and will reduce the parasitic loss to the ASEprocess, thereby preserving more available energy for the output pulseof the APL.

The trigger module 110, via its trigger connection (or path) 108,transmits photons into the storage medium 113 in order to rapidlyrelease the stored energy in the storage medium through stimulatedemission. The power and shape of the trigger pulses (or signals) aresuch that the initial portions of the trigger pulse create a cascadingenergy release and deplete the energy out of the APL 100 such that thetrailing portions of the trigger pulse are not significantly amplified.Trigger connection 108 may have one or more optical elements forcommunication of pulsed optical beam(s) to the storage medium 113, suchas fiber optics, beam shaping lens(es), or other optical components,which may depend on the particular type of storage medium 113. Thewavelength of light emitted by the trigger module 110 is thereforeselected to be the wavelength of light desired (in the range ofwavelengths of possible emission from storage medium 113) to be outputby the APL module 100 in the output beam 109. The trigger module 110 maybe composed of one or a plurality of trigger sources that emit light,such as, but not limited to, a flash lamps, LEDs, lasers, electronicsthat produce electrical arc discharges, or fluorescent lamp. However,for applications requiring an output beam 109 that is reasonablycoherent, such as LIDAR and medical ablation applications, LEDs andlasers are preferential for the trigger sources contained within thetrigger module.

Modules 110-112 and storage medium 113 may be provided in a housing ofthe APL 100 having a window or port through which output optical pulses109 are emitted. Such housing may have boards for supporting theelectronics of module 110 and sources for modules 110 and 112, andconnectors or lines for power and control signals 103-106, and opticalconnections 107-108. Such APL 100 may be a subassembly in a system forproviding a source of high power short optical pulses in applications,such as LIDAR, remote sensing, laser altimeters, laser range finding,medical procedures, or other optical applications.

In FIG. 2 an illustrative example of a timing diagram for the APLmodules and APL output power is presented. As diagrammed in FIG. 2 andin reference to the schematic of FIG. 1, to generate a train of opticaloutput pulses 109 from the APL of period T_(period) (and therefore arepetition rate of 1/T_(period)) the control module 111 directs the pumpmodule 112 to begin pumping energy at discrete intervals T_(period) fora temporal duration of T_(pump), such that a series of pump pulses 200is generated. For example, at a time T_(o) energy begins flowing fromthe pump module 112 into the storage medium 113 via the pump connection107. At time T_(o) the output power of the storage medium begins toclimb slowly due to the finite leakage of light as no storage medium is100% efficient at storing energy. For a storage medium 113 describedearlier that is a laser media, this leakage of energy is generally fromspontaneous emission due to atoms decaying from an excited or invertedpopulation state down to a ground state as will described in more detailin connection with FIG. 4. The plotted power of the pump pulse 200 mayrepresent power in the form of conductive heat, radiative energy, light,electricity, or any combination thereof that is being directed into thestorage medium 113. At a time T₁ (equal to T₀+T_(pump)) the pump module112 stops sending energy into the storage medium, and the trigger module110 is turned on such that a trigger pulse 201 of optical energy isgenerated with a FWHM equal to T_(trigger). In practice, the triggermodule 110 may be turned on before the pump module 112 is turned off.Alternately the trigger module 110 need not be turned on immediatelyafter the pump module 112 is turned off, but if the delay between thetwo said events is minimized, the APL will be more efficient due to thenon-zero energy leakage rate that exists for any practical storagemedium. Due to the photons contained within the leading edge of thetrigger pulse 201 that begins flowing through the storage medium 113 viathe trigger connection 108, the storage medium 113 begins to dump itsstored energy through stimulated emission. As the initial photons of thetrigger source generate new photons via stimulated emission within thestorage medium 113 these new photons propagate further into the storagemedium and generate additional coherent photons via stimulated emission.The result, not dissimilar to an avalanche, is a wall of photons thattravel through the storage medium 113, constantly amplifying until themaximum amount of power per unit length of the storage medium is dumped.Due to the avalanche effect being generated by the leading edge of thetrigger pulse, the output power of the APL 100 does not necessarily peakat the peak power of the trigger pulse. As illustrated in FIG. 2, thepeak of the output power pulse 202 of FIG. 2 (109 in FIG. 1) occurs at atime T₂, while the peak power of the trigger pulse may occur at latertime T₃. Also, the peak output pulse P_(max) can be significantly largerthan the peak power P_(t) of the trigger pulse. In fact, depending uponthe storage capacity of the storage medium 113, the ratio P_(max)/P_(t)can exceed several orders of magnitude. Due to the avalanche effectdraining the stored power of the storage medium 113, only the leadingedge of the trigger pulse of width T_(trigger) gets amplified. Theremainder of the trigger pulse passes through the storage medium 113with little or no amplification. The aforementioned avalanche effectresults in an output pulse 202 (109 in FIG. 1) having a FWHM T_(out)that is significantly narrower than the trigger pulse 201 used togenerate the output pulse 202. In this manner, for example, a nanosecondwide output pulse can be generated with a microsecond trigger pulse andas such the present invention differs dramatically from an opticalamplifier that otherwise would exhibit a FWHM of an output pulse beingsubstantial similar to that of an input pulse.

As described earlier, the control module 111 controls the repetitionrate of the output beam pulses 109 by controlling the repetition rate ofthe pulses generated by the pump module 112 and trigger module 110.Although the pump module 112 signal in FIG. 2 is illustrated as aperiodic event, for cases in which the pump sources of module 112require an unacceptable time to power up and power down or for cases inwhich the repetition rate of the desired output pulses 109 are highenough that as soon as the trigger pulse fires the pump pulse must beturned on again, in such scenarios, the pump module 112 may be designedsuch that it supplies energy to the storage medium 113 in a continuousbeam. In such situations, it is the electronics of the control module111 that controls the timing of the trigger module's pulses thatdictates the timing of the APL's output pulses 109. For control of theoutput energy and power of the APL's output pulses 109, the controlmodule 111 regulates the amount of energy pumped into the storage mediumby pump module 112 before the trigger pulses are fired by the triggermodule 110 into the storage medium 113. The control module 111 may alsochange the shape of the trigger pulse so that the energy containedwithin the storage medium 113 is more or less rapidly discharged. Forexample, trigger module 110 may have one or more semiconductor lasers,in which the level of drive current applied thereto (in response tocontrol signals 106) can adjust pulse shape, such as the rising edge oftrigger pulses. For example, for an APL 100 operation in the C or Lcommunication band (i.e., approximately 1520 to 1650 nm wavelengths),trigger module 110 may have trigger sources that are semiconductorcommunication lasers. Trigger lasers can be directly controlled at ratesof 2.5 GHz, such as the C-band or L-band lasers available commerciallyfrom CyOptics of Breinigsville, Pa., USA, or JDSU of Milpitas, Calif.,USA. For faster modulation and shaping of the trigger laser pulse (e.g.,10 and 40 GHz), the trigger module 110 may have semiconductor lasersthat are integrated with a high-speed optical modulator such aselectro-absorptive modulators operating at 10 GHz (e.g., as available byCyOptics), or a laser with constant optical output may be modulated by alithium niobate optical modulator, such as available from JDSU.

For certain applications, the APL output pulses 109 emitting in thegeneral 1.55 μm portion of the spectrum are desirable, as thiswavelength region represents a spectral window where moisture in the airdoes not absorb and the spectral band is considered to be eye-safe incomparison to shorter wavelengths such as the 1.06 μm wavelength of aYAG laser or the 0.532 μm wavelength of a doubled YAG laser. As such,wavelengths in the 1.55 μm portion of the spectrum have applications inremote sensing, LIDAR, laser range finding, and laser altimetry. Suchspectral band also has applications in the medical industry for thetreatment of skin pigmentation including port wine stains.

Referring to FIG. 3, one embodiment of the APL 100 is illustrated forpulsed laser applications requiring wavelengths in the general 1.55 μmportion of the spectrum. In this embodiment, the storage medium 113contains an optical fiber. Preferably, such fiber is Erbium-doped silicafiber which is commercially available from vendors such as Corning,Inc., of Corning, N.Y., USA, or OFS Fitel Lucent, of Somerset, N.J.,USA. Erbium-doped silica can serve as an efficient storage medium andhas been used in the fabrication of optical amplifiers in the prior art.As such, such erbium doped optical fiber can serve as one component ofthe APL 100. The energy levels of the fiber are depicted in FIG. 4. Theenergy diagram represented in FIG. 4 is a 3-level laser system whereinatoms at the ground state labeled <0> make a transition 400 to anexcited upper state labeled <2> via a pump photon 406. For Erbium-dopedsilica, the ground state <0> is composed of 8 Start split states and theenergy difference between levels <2> and <0> can be efficiently pumpedwith a pump module emitting photons with wavelengths of approximately980 nm.

Due to the manifold of Stark split states comprising the levels <0> and<1>, and to the small energy difference between the Stark splitsub-levels compared to the room temperature thermal energy of kT˜1/40electron volt the populations of the sub-levels of the manifolds <0> and<1> are distributed according to the Boltzmann thermal distribution. Asa result, in un-excited Er-doped glass, absorption is most favoredbetween the lowest energy sub-level (most heavily populated) of the <0>state and the least populated (highest energy sub-level) of the <1>state.

Conversely, emission is favored between the lowest sub-level of <1> andthe upper sub-level of <2>. This results is a significant difference inthe absorption and emission spectrum of the Er-doped glass, allowingoptical pumping directly from <0> to <1> at wavelengths in the range of1480 nanometers.

Although atoms at the excited state <2> can spontaneously decay 404 backdown to the ground state and reradiate an incoherent pump photon 409 ofapproximately the same wavelength as that emitted by the pump module,the time constant for said transition T₂₀ is significantly longer than ananosecond. The more likely transition is the transition from theexcited state <2> to an inverted state <1> as marked by the arrow 401.The transition process is nonradiative and results in almost all of theatoms pumped out of the ground state <0> to populate the inverted state<1>. Atoms remain in the inverted state <1> for a significant amount oftime since the spontaneous decay transition from this state, labeled as405, is on the order of τ₁₀˜10 ms. It is the spontaneous decay of atomsfrom the inverted state <1> to the ground state <0> producing incoherentphotons 410 that produces a non-zero output power from the storagemedium 113 that is represented in the output power graph of FIG. 2during the time interval between T₀ and T₁. Therefore, to pump themaximum amount of energy into the storage medium 113 that containsErbium-doped silica, a pump module 112 is provided such that the pumppower P_(p) entering the storage medium 113 is sufficiently high thatthe pump pulse 200 can have a length T_(pump) that is shorter than theinverted states transition time of 10 ms, while still exciting all ofthe ground state <0> atoms into the inverted state <1>. Although asingle fiber tap from the pump module 112 is shown being attached to thestorage medium via the wavelength division multiplexer 303 and the fibertap 309, multiple fiber taps from the pump module 112 to the storagemedium 113 may be used in order to efficiently pump a fiber spool ofsignificant length in under 10 ms. With atoms that are in the invertedstate <1>, they can be brought down to the ground state <0> viastimulated emission. An incident photon 407 (such as from a triggermodule 110) will stimulate an atom in the inverted state to drop down tothe ground state <0> and in the process emit another photon 403 that isat the same wavelength and in phase with the incident photon 407. Theincident photon 407 will continue propagating through the storage medium113 as photon 408 and both photons 403 and 408, if encountering otheratoms in the inverted state can generate additional photons throughstimulated emission, thereby resulting in an amplification of theinitial photon 407. For Erbium-doped fused silica, due to the Starksplit levels of the inverted state <1> and the Stark split levels of theground state <0>, the wavelength of the incident photons capable ofproducing stimulated emission can range in wavelengths fromapproximately 1520-1650 nm.

The Erbium-doped silica providing storage medium 113 in FIG. 3 may besubstituted by a silica media doped with other species of rare earthions, in order to operate at a different wavelength range of the opticalsignal. It may also be substituted by a fiber having additional dopingions, such as Ytterbium, which allow the use of a pump laser having adifferent wavelength than those listed above. Further, it may besubstituted by an optical fiber in which the process of Stimulated RamanEmission gain is used to amplify the signal, allowing a very wide rangeof choice of signal wavelength by selection of appropriate pump laserwavelength. Other glass hosts, such as phosphate glasses, which have theadvantage of accepting a much high doping concentration of Erbium ions,may also be used. The optical fiber may be configured as a claddingpumped fiber in which the pump laser is injected into the optical fiberin a region that forms the cladding for the area in which the opticalsignal travels. Other variants of optical fiber having differentcompositions, dopings, or pump methods may also be used in addition tothose specifically described above.

Continuing to refer to FIG. 3, the pump module 112 for an APL 100containing Erbium-doped silica as its storage medium 113, containspreferentially a single or a plurality of 980 nm lasers. Although otherpump sources such as a flash lamp can be used, a flash lamp, thoughinexpensive, can be used only for applications where the repetition rateof APL output pulses 306 is on the order of 1 Hz due to the rechargetime required by the flash lamp. The pump module 112 is coupled via anoptical fiber 301 to a wavelength division multiplexer (WDM) 303 thatcombines the radiation of the trigger module 110 that is also fibercoupled via an optical fiber 302 into a common third fiber 309 that maybe fusion spliced to the input end face of a spool of Erbium-dopedsilica fiber of FIG. 3 providing storage medium 113. Such arrangementmay be replicated using free-space optics for coupling the pump sourceand trigger source to the storage medium 113, though if the Erbium-dopedsilica of the storage medium is in the preferred form of an opticalfiber, it may be advantageous to use fiber-pigtailed lasers for the pumpsources and the trigger sources and to used a fused fiber WDM, such asis sold by Sifam Fiber Optics of Torquay, United Kingdom. TheErbium-doped silica fiber may be replaced with an Erbium-doped bulksilica material, so that no fibers are utilized.

The pump module 112 of FIG. 3 is capable of pumping the Erbium-dopedsilica fiber via the WDM 303 and the input fiber 309, and the triggermodule 110 is capable of triggering the release of the energy via theWDM 303 and the input fiber 309.

For example, APL 100 of FIG. 3, using the parameters of the timingdiagram of FIG. 2, may have P_(p) and P_(t) of 200 milliwatts and 1-2milliwatts, respectively, with a T_(pump) of 3 mS, a T_(trigger) ofrange of 1 ns to 1 ms, produces P_(max) of 100 kW watts having aduration of 0.5 to 1 ns (100 microJoules). The erbium fiber is capableof storing in this example 400 microJoules of optical energy. In thisexample, using fiber fusion splicing for assembling components, andsupplying the necessary electrical drive current to the pump laser andthe trigger laser, the APL 100 of FIG. 3 may have the following exampleof components to achieve such parameters: storage medium 113 may be anErbium fiber, model MP-980 type, 30 meters, available from OFS-Fitel ofSomerset, N.J., USA; pump module 112 may be a 980 nm pump laser, model2600 series, ˜200 mW, available by JDSU, of Milpitas, Calif., USA; thetrigger module 110, a D572 type laser, 2 mW, wavelength 1530-4560 nm,available from CyOptics of Breinigsville, Pa., USA; and beam combiner303 may be a 980 Fused Pump-Signal WDM, available from Sifam, Torquay,UK. One application of the APL 100 of such example is as a high powerpulse light source for a laser range finder providing an output pulserate of 300 Hz.

The trigger module 110 may contain one trigger source or a plurality oftrigger sources. The trigger sources need not be all at the samewavelength nor have their respective trigger pulses synchronized. By wayof example, the APL 100 may be programmed via control module 111 to emitvia stimulated emission a specific wavelength within the band ofwavelengths that the storage medium 113 is capable of outputting (e.g.,approximately 1520-1650 nm the case of Erbium-doped silica). Thereforewithin the trigger module 110 a trigger source with the desiredwavelength will be turned on to produce an APL output pulse of the samedesired wavelength, while trigger sources of other emission wavelengthswould be turned off. Additionally, the trigger module may containtrigger sources that are capable of emitting over a range ofwavelengths, such as a tunable semiconductor laser, such as for example,made by Intel, of Santa Clara, Calif., USA; Bookham, of San Jose,Calif., USA; or JDSU, of Milpitas, Calif., USA. Such APL incorporatingmulti-wavelength trigger sources may also be programmed to emitalternating pulses of different wavelengths. In other words, alternatingpulses of the trigger pulse diagrammed in FIG. 2 may be differentwavelengths. Control signals 106 to the trigger module 110 control thewavelength of multi-wavelength trigger sources and/or tunable triggersources.

Optionally, a dispersion-controlling optical element 307 may be providedfor receiving optical pulses outputted from storage medium 113, via anoptical fiber 304 or other optical means, to change the temporal width(or shape) of such optical pulses. This may be useful if the triggermodule 110 contains spectrally broadened optical sources, such assuper-luminescent LEDs, as such the outputted light pulses from thestorage medium 113 will also be spectrally broadened. For example, thetrigger pulses may have a substantially continuous optical spectrum witha spectral width of at least ten nanometers, or less than one nanometer.The dispersion controlling element 307 may for example be a spool ofdispersive fiber if the desire is to broaden the pulse width, or a spoolcontaining dispersion compensating fiber if the desire is the narrow thepulse width. The dispersion-controlling element 307 may also be adiffraction grating to perform pulse broadening or compression.

To control the cross-sectional shape of the output pulse 306 of the APL100, optional beam-shaping optics 305 can be incorporated into the APL100. By way of example, beam shaping optics 305 may be a simple fiberand fiber connector such that the APL can be coupled to otheropto-electronic modules using standard FC or SC or other connectors. Asa further example, beam-shaping optics 305 may consist of free-spacelenses, mirrors, or other optical elements that shape the output beaminto a beam of a specific diameter and reasonably collimated, such thatit is capable of transmitting distances ranging from several meters toseveral kilometers, such as for the purposes of laser range-finding. Asan additional example, beam-shaping optics 305 may be used to createfocused beam or an array of focused beams for the purposes of lasermachining or epidural treatment as part of a medical procedure.

Since the APL is capable of generating high-power pulses, one issue withthe use of a fiber spool of FIG. 3 as the storage medium 113 is theability to couple light out of such fiber without incurring opticaldamage at the fiber interface where the optical energy emerges into air.Referring to FIG. 5, the end of the optical fiber of FIG. 3 providingthe storage medium 113 is denoted as reference numeral 500. Atransparent bulk optical element 504 is attached, such as fused orbonded, to the output end face of the optical fiber 500 in order toreduce the chance of optical damage. The optical pulse 503 emerging fromthe storage medium 113 is concentrated within the core 502 of the fiberdue to the index of refraction difference between the core and the fibercladding 501. In the absence of an attached transparent bulk optic 504,the optical pulse 503 has a cross-section of diameter d₁ which by way ofexample for a multi-mode fiber may be approximately 50 μm and for asingle-mode fiber may be approximately which due to the small size of d₁can result in high optical power densities. By attaching a transparentbulk optic element 504, the optical pulse first become an expanding beam506 due to diffraction before it emerges from bulk optic 504. Thetransparent bulk optic 504 preferably has an index of refraction thatnearly matches that of the optical fiber 500 to minimize backreflections and the attachment is preferentially done with low-defectmaterials, such that optical damage does not occur between said bulkoptic and the optical fiber 500. By way of example, the optical fiber500 may be a fused silica-based optical fiber, the bulk optic 504 afused silica window of thickness L, and the attachment of bulk optic 504to optical fiber 500 performed through the use of a laser fusionprocess. The thickness of the bulk optic L is chosen according to howmuch the optical power density of the optical fiber end face must bereduced in order to minimize the risk of optical damage. With thetransparent bulk optic 504, the optical power density will be reduced by

$\begin{matrix}{{( \frac{d_{2}}{d_{1}} )^{2} \approx \lbrack {1 + {2\frac{L}{d_{1}}{\tan\lbrack {\sin^{- 1}( \frac{NA}{n} )} \rbrack}}} \rbrack^{2}},} & (1)\end{matrix}$where d₁ is the width of the output beam 503 in the optical fiber, NA isthe numerical aperture of said fiber and n is the index of refraction ofthe transparent bulk optic. For example, taking NA=0.13, n=1.46 (fusedsilica), L=5 mm, and d₁=0.010 mm, using Eq. (1), the optical powerdensity is reduced by a factor of 32,000.

As depicted in FIG. 5, the expanding beam 507 emerging from thetransparent bulk optic 504 encounters beam shaping optics 305 thatsubstantially collimates the optical output beam 306. In general, thebeam shaping optics 305 are optional, but when present they may, by wayof example, may be used to alternately focus or diverge the beam 507.

Optionally, to suppress ASE in the APL 100, optics 305 or 307 may alsorepresent an optical isolator in the path of the output pulse from thestorage medium. The optical isolator is substantially transparent forlight passing in one direction, but substantially opaque to lighttraveling in the reverse direction (commonly a factor of 1000 to 10,000difference). An optical isolator will prevent backward traveling light(which would only be ASE) from parasitically depleting the gain of thestorage medium. Also, to prevent ASE propagation optical filters may beprovided in the path of the trigger pulses to the storage medium or atthe input to storage medium, such optical filters transmit thewavelength(s) of the trigger pulses selected to be outputted by thetrigger module, but substantially attenuate wavelengths which departfrom the trigger pulses. Use of such optical isolator or optical filterscan prevent the buildup of ASE in both the forward and the backwarddirection, allowing a greater amount of energy to be stored in theoptical storage medium.

For certain applications, polarization control of the APL's outputpulses (or beam) 109 (FIG. 1) or 306 (FIG. 3) may be provided by eitherusing trigger pulses of defined polarization, and/or using polarizationcontrol element(s) to change (or maintain) polarization of output pulsesfrom storage medium 113. In this manner for example, polarization may becontrolled with regards to TE or TM, or circular or linear,polarization. Polarized trigger pulses may be achieved by using intrigger module 110 that emits optical trigger pulses of a definedpolarization by using polarized optical sources. For example, a triggermodule 110 that incorporate trigger sources that are lasers or LEDs asthese optical sources may be obtained as polarized optical sources.Since the storage medium 113 releases its energy via stimulated emissiondue to the trigger optical pulse received, the radiation emitted by eachatom in the storage medium dropping from a higher state to a lower statewill have the same polarization as that of the incoming trigger opticalpulse. However, any reflections the trigger pulse encounters beforeinducing stimulated emission or after, may alter the polarization stateof the final output optical pulse relative to the initial triggeroptical pulse. This polarization altering or depolarization phenomena isespecially true for optical fibers that can bend in arbitrarydirections. For an APL using optical fibers for interconnection ofmodules and/or for use as the storage medium 113 itself, it is thereforepreferential that the optical fibers within the APL 100 are polarizationmaintaining (PM) fibers. As will be described below, a polarizationcontrol element 607 may also be used to control polarization of outputpulses 109.

Referring to FIG. 6, the APL 100 having output pulses 109 of a certainpolarization is shown further having a module for enabling heterodyne orhomodyne detection for improved detection sensitivity. For example,various types of vegetation can be distinguished using polarimetricremote sensing, such as described in K. P. Papathanassious and S. R.Cloude, “Single Baseline Polarimetric SAR Interferometry,” IEEETransactions Geoscieve and Remote Sensing 39, 2352, (November 2001).Polarization control also allows heterodyne or homodyne detection inremote sensing for the purposes of increasing the SNR (Signal to NoiseRatio) of the sensor system due to the coherent construction anddestruction of the reflected radiation with a control signal. Theoptical trigger pulse provided by optical connection 108 is shown inFIG. 6 emerging from the trigger module 110 with an arrow 611 toindicate that it is polarized within the plane of the figure. A smallfraction of optical trigger pulse is directed using beamsplitter 600along optical connection (or path) 605 to an interferometric detectionmodule 606. Preferably, such the small fraction of the trigger pulse hasgreater optical power that the returned optical signal 604, in order tohave a large increase in the SNR. The remainder of the trigger pulseenters into the storage medium 113 via path 108 a and generates anoptical output pulse 608 that is also polarized within the plane of thefigure as denoted by arrow 612. Optionally, a polarization controlelement 607 is provided in order to alter the polarization of the outputoptical pulse 608, if needed. Such polarization control element 607 maybe static, such as for example, a birefringent waveplate made of quartz,birefringent polymer, MgF2 or other substantially transmitting opticalmaterial, or may be dynamic, such as for example, a static waveplatemounted on an automated rotation stage, or an electro-optic crystal thatcan be dynamically addressed to change the polarization of optical pulse608. In the case of a dynamic polarization control element 607, thecontrol module 111 may have a power and signal lines (not shown) toelement 607 to control the polarization state of the output pulse. Theoutput pulse after having its polarization (TE or TM) altered bypolarization control element 607 may optionally be beam shaped by beamshaping optics 305 to provide the desired output optical pulse 109.

In the example of FIG. 6, the polarization of output beam 109 wasrotated 90 degrees from its original orientation of optical pulse 608emitted from storage medium 113, such that it is polarized perpendicularto the plane of the figure. In general, however, the polarization mayremain linear and rotated by a different angle, or may be converted intocircular or elliptical polarization by polarization control element 607.If needed, additional polarization control element(s) may be provided inthe path of the output pulse to provided the desired polarization ofoutput optical pulse 109. The output pulse 109 emitted by the APL 100for remote sensing applications, such as LIDAR, laser altimetry, andlaser range finding directed at and reflect off of a distant object (notdrawn). The returned optical signal is denoted by arrow 604, and iscollected by optics 610 and coupled into the interferometric detectionmodule 601. Within the detection module, the reflected optical beam iscombined using mirror 602 and beamsplitter 601 with sampled triggeroptical beam from connection or path 605. The resulting interfered beamsis detected by optical detector 603, such as a CCD array orphotodiode(s).

The interferometric detection module 606 may alternatively be placedexternally to the APL 100 while achieving the same function. Theinterferometric detection module 606 illustrated is one possible methodfor combining two optical beams. Other means or methods as typicallyused in interferometry for combining two optical beams may also be used.

If a trigger optical pulse emerging from trigger module 110 has arelatively short coherence length, delay optics 609 may be provided.Such delay optics 609 may be for example, a spool of optical fiber. Theoptical delay such delay optics 609 impart onto the sampled triggeroptical beam along connection or path 605 is approximately matched tothe round trip optical path length that the APL output pulse 109receives before being detected in reflected optical signal 604 at thedetection module 606. Since the exact time of flight of a returned APLoutput pulse 109 cannot be predicted, it is advantageous that alow-level constant signal be emitted by the trigger module 110 such thatthere is always a signal along the sampled trigger optical connection orpath 605 that can be interfered with the returned optical signal 604.Thus, the small portion of trigger optical pulses passed to detectionmodule 606 persists in time at least until after the return of thereturned optical signal 604.

Alternatively for heterodyne detection, the beamsplitter 600 of FIG. 6may be removed, and only a single output beam 109 having controlledpolarization provided. Referring to FIG. 7, such an alternativeheterodyne detection using APL 100 is illustrated. A second lightsource, termed the heterodyne source 700 is present in detection module606. This heterodyne source interferes with return signal 604 ontodetector 603. As such, heterodyne source 700 must be locked inwavelength with respect to the APL output pulse 109 which puts therequirement upon trigger module 110 that the wavelength emitted by thetrigger connection 108 must be wavelength stabilized. Such wavelengthstabilization is straightforward for certain lasers that may be used astrigger sources, for example HeNe laser, which do not have a strongtemperature dependence upon wavelength. For other laser sources that aresignificantly more temperature dependent and fabrication dependent, suchas semiconductor lasers, the use of Bragg gratings, Fabry Perot feedbackcontrol, and thermo-electric coolers (TECs) may be used to lock thewavelengths to accuracies less than 5 GHz in frequency. Preferentially,the heterodyne source 700 emits at substantially the same wavelength asthat of the trigger module such that the temporal dependence of theinterference on detector 603 is minimized. Both the heterodyne source700 and the interferometric detection module 606 may be provided withinthe APL 100.

The output pulse 109 parameters of APL 100 vary dependent upon differentoptical applications. For example, in the case of remote sensing using alaser range finder to track an object moving with respect to the finder,the pulse energy, pulse repetition rate, and pulse length required ofthe APL output pulse will depend upon the maximum range of use, thelongitudinal distance accuracy of the laser range finder, and therelative speed of the laser range finder with respect to the objectunder observation. Generally, the pulse width should be equal to thetime it takes light to travel the distance specified by the laser rangefinder's longitudinal accuracy. The APL pulse repetition rate is suchthat the object under observation does not move appreciably (i.e., lessthan the longitudinal distance accuracy specification for the laserrange finder) during the time between pulses. Alternatively for laserrange finders that can rapidly calculate trajectory of objects underobservation, the trajectory of the object under observation cannotchange appreciably (i.e., the extrapolated position of the object underobservation must differ from the actual object location by less than thelongitudinal distance accuracy specification for the laser range finder)during the time between pulses. The APL power per output pulse is suchthat the sufficient power returns to the detector of the laser rangefinder. This is a function of the signal-to-noise ratio (SNR) required,the detector noise equivalent power (NEP), the maximum distance of laserrange finder operation, the reflectivity and scatter profile of theobject under observation, the collection aperture of the laser rangefinder's optics, and environmental conditions including fog, rain, andsandstorms.

One limitation on the ability of laser sources conventionally used inlaser rangefinders to identify a target object at a large distance, anda limit on the ability to discriminate between multiple targets, is theoptical power of the returned optical signal. Two ways to increase theoptical power of the returned optical signal are (1) to transmitincrease the power of the output beam 109, or (2) to collect morereturned optical signal by increasing the size of the collecting optics.Therefore, as APL 100 more efficiently outputs energy from the storedmedium 113, the application of an APL 100 with optical heterodyne orhomodyne detection in a laser rangefinder may improve performance with asignificant advantage in terms of cost, size and weight. For example,the APL 100 may by incorporated into laser range finding devices orsystems, such as available from ALST of Orlando, Fla., USA or Nikon ofJapan.

From the foregoing description, it will be apparent that there has beenprovided an improved apparatus and method for generating short opticalpulses, and such apparatus and method provides a high power pulse lasersource for various optical applications. Variations and modifications inthe herein described apparatus and method in accordance with theinvention will undoubtedly suggest themselves to those skilled in theart. Accordingly, the foregoing description should be taken asillustrative and not in a limiting sense.

1. An apparatus for generating short optical pulses comprising: astorage medium capable of storing optical energy; a first module forpumping optical energy into the storage medium to energize the storagemedium; a second module for delivering at least one optical pulse to thepumped optical storage medium, in which rather than pumping said storagemedium said optical pulse triggers a substantial depletion of alldischargeable energy from said pumped optical storage medium in anoutput optical pulse from the storage medium, wherein said optical pulserepresents a trigger pulse, said trigger pulse is polarized, and saidoutput optical pulse has a greater power and narrower full width at halfmaximum size than the trigger pulse that triggered the output opticalpulse; means for controlling the polarization of said output opticalpulse; optics for receiving a return optical signal in response to saidoutput pulse; and a third module for detecting said return opticalsignal, wherein a portion of said trigger optical pulse persists in timeat least until after the return of the returned optical signal.
 2. Theapparatus according to claim 1 wherein said trigger optical pulse has arising edge which triggers a cascade release of energy in an outputoptical pulse from the storage medium by substantially discharging theenergy stored by said storage medium.
 3. The apparatus according toclaim 1 further comprising a fourth module controlling the operation ofthe first and second modules to effect characteristics of the outputoptical pulse.
 4. The apparatus according to claim 1 wherein saidtrigger pulse is of one or more wavelengths, and the output opticalpulse is of the same said one or more wavelengths as the trigger opticalpulse that caused the output optical pulse.
 5. The apparatus accordingto claim 1 wherein said output optical pulse has a reduced temporalduration due to strong saturation of the gain by the leading edge of thetrigger optical pulse that caused said output optical pulse.
 6. Theapparatus according to claim 1 further comprising a housing having atleast said storage medium, said first module, and said second module. 7.The apparatus according to claim 1 wherein the output optical pulse hasa peak which is duration of less than one microsecond.
 8. The apparatusaccording to claim 1 wherein the output optical pulse has a peak whichis duration of less than ten nanoseconds.
 9. The apparatus according toclaim 1 wherein said first module pumps optical pulses into the storagemedium to energize the storage medium.
 10. The apparatus according toclaim 1 wherein said first module pumps a continuous beam into thestorage medium to energize the storage medium.
 11. The apparatusaccording to claim 1 wherein said second module represents one or moresources for delivering said optical pulse.
 12. The apparatus accordingto claim 1 wherein said storage medium is one of an optical fibercapable of optical energy storage.
 13. The apparatus according to claim1 wherein said storage medium is an optical fiber of glass material ofone of silica glass, phosphate glass, fluoride glass, or chalcogenideglass, in which at least a portion of the said glass material has aconcentration of one of or more of Erbium, Ytterbium, Neodymium, orPraseodymium.
 14. The apparatus according to claim 13 in which saidportion of the said glass material forms a continuous filament orchannel between ends of the optical fiber.
 15. The apparatus accordingto claim 1 wherein said storage medium is an optical fiber composed of acrystalline material.
 16. The apparatus according to claim 15 whereinsaid crystalline material is of photonic crystal composition.
 17. Theapparatus according to claim 1 wherein said storage medium representsone of a gas mixture, glass, crystal, or semiconductor, or a liquid. 18.The apparatus according to claim 1 further comprising at least oneoptical element for cross-sectional shaping of said output optical pulseoutputted from the storage medium.
 19. The apparatus according to claim1 wherein said storage medium is an optical fiber having an input facefor receiving said pumping optical energy and said trigger pulse . 20.The apparatus according to claim 1 wherein said storage medium is anoptical fiber having an output face for outputting said output opticalpulse, and said apparatus further comprises an optical element attachedto said exit face which prevents damage to the exit face of the opticalfiber.
 21. The apparatus according to claim 20 wherein said opticalelement is a transparent block having an index of refraction thatsubstantially matches the index of refraction of the optical fiber. 22.The apparatus according to claim 1 wherein said second module comprisesone or more sources for producing said trigger optical pulse having oneor more wavelengths, in which said one or more sources are tunable, andsaid output optical pulse is of one or more wavelengths in response tosaid tuned one or more wavelengths of said one or more sources.
 23. Theapparatus according to claim 1 further comprising at least one opticalelement providing a dispersive medium for broadening the shape of saidoutput optical pulse outputted from the storage medium.
 24. Theapparatus according to claim 23 wherein said trigger optical pulse has asubstantially continuous optical spectrum.
 25. The apparatus accordingto claim 23 wherein said dispersive medium is one of a diffractiongrating or a dispersive optical fiber.
 26. The apparatus according toclaim 1 wherein said storage medium outputs a plurality of ones of saidoutput optical pulse each in response to one of said trigger pulse beingdelivered to said storage medium when pumped with optical energy by saidfirst module.
 27. The apparatus according to claim 1 wherein saidapparatus is part of a system utilizing said output optical pulse forone of remote sensing, LIDAR, laser altimetry, or laser range finding.28. The apparatus according to claim 1 wherein said apparatus is part ofa system for application of said output optical pulse to the body of apatient to enable a medical procedure.
 29. The apparatus according toclaim 1 further comprising means for enabling said output pulse to beemitted from said apparatus and collected by said third module forremote sensing in accordance with homodyne or heterodyne detection. 30.A method for generating short optical pulses using a storage mediumcapable of storing optical energy comprising the steps of: pumping oneof optical pulses or continuous beam into the storage medium to energizethe storage medium; delivering one or more polarized trigger opticalpulses to the pumped optical storage medium in which each of the triggeroptical pulses without pumping the storage medium triggers a substantialdepletion of all dischargeable energy from said pumped optical storagemedium in an output optical pulse from the storage medium having agreater power and narrower full width at half maximum size than thetrigger optical pulse that triggered the output pulse; controlling thepolarization of said output optical pulse; and receiving a returnoptical signal in response to said output pulse, wherein a portion ofsaid trigger optical pulse persists in time at least until after thereturn of the returned optical signal.
 31. The method according to claim30 wherein said one or more trigger optical pulses each have a risingedge which triggers a cascade release of one of said output opticalpulse from the storage medium by substantially discharging the energystored by said storage medium.
 32. The method according to claim 30further comprising the step of controlling the operation of the firstand second modules to effect characteristics of the output opticalpulses.
 33. An avalanche laser comprising: a storage medium capable ofstoring optical energy; means for pumping optical energy into thestorage medium to energize the storage medium; and means for inputtingto the pumped optical storage medium at least one polarized opticalpulse having one or more characteristics which triggers a substantialdepletion of all dischargeable energy from said pumped optical storagemedium in an output optical pulse having a greater power and narrowerfull width at half maximum size than the inputted optical pulse thattriggered the output pulse, wherein said output pulse represents anamplified version of only a portion of said input pulse that triggeredthe output pulse; means for controlling the polarization of said outputoptical pulse; means for receiving a return optical signal in responseto said output pulse; and means for detecting said return opticalsignal, wherein a portion of said trigger optical pulse persists in timeat least until after the return of the returned optical signal.
 34. Thelaser according to claim 33 wherein said one or more characteristics arethe power and shape of the inputted optical pulse.
 35. The laseraccording to claim 33 wherein said operation of said pumping means andsaid inputting means are in timed relationship with each other.
 36. Thelaser according to claim 33 wherein said storage medium outputs aplurality of ones of said output optical pulse each in response to aplurality of ones of said optical pulse being inputted to said storagemedium when pumped with optical energy.